Sunday, May 1, 2016

Endogamy In Action; Sibling Edition

Earlier, I looked at how endogamy made me share much more DNA with known relatives than would be expected. While this can help to guide those with endogamous backgrounds to know what varying amounts of shared DNA may mean about actual relationships, it's important to remember that the amount can significantly vary by individuals. Each of my parents has a brother who has tested, and my parents have both been tested as well. While each of my parents and their brothers are related to other tested relatives in the same way, in practice, they sometimes share a very different amount of DNA with known relatives.

Name

Actual Relationship

Shared cM w/mom

Shared cM w/uncle

ISOGG average

Comments

Ruth

Aunt

1771.4380

1725.0190

1700.00

Scott

1C

1013.0784

732.1877

850.00

Ken

2C1R

281.4508

243.9142

106.25

Myron(2nd cousin to Ken; 1/2 second cousin to Sue)

2C1R

181.4029

232.4263

106.25

Sara(Myron's niece)

3C

95.9599

158.6909

53.13

Sue(1/2 second cousin to Ken and Myron)

Half 2C1R

134.2947

123.5351

53.13

Judith

3C

224.05

138.70

53.13

Jonathan (Judith's son)

3C1R

160.0293

79.4094

26.56

Pat

2C

181.9369

221.3467

212.50

Ben

2C1R, 3C1R

127.7907

143.6804

106.25+

Has 2 Supkoff great grandparents

Table showing shared DNA between my mother and maternal uncle and other known relatives

In the above chart, the columns represent the following:

The first column is the relative who has tested

The second column is the testee's known relationship to my mother and her brother; xCyR means that the person is an xth cousin, y times removed.

The third column is how many cM of DNA my mother shares with the individual

The fourth column is how many cM of DNA my maternal uncle shares with the individual

Finally, the sixth column has additional information about the individual which may be relevant.

Even at the first cousin level, the difference in amounts of shared DNA is pretty extreme--and likely not due (much) to endogamy at that point. My uncle and his first cousin Scott share 723cM--less than the ISOGG expected amount, while my mother shares quite a bit more--1013cM.

All of the other relatives on my mother's side share more DNA with both my mother and uncle than the ISOGG average (yay endogamy!), but there are still huge differences in shared amounts between my mother and uncle.

For example, cousin Jonathan is my mother's third cousin once removed, so he'd be expected to share about 26.56cM with the siblings. My uncle shares much more DNA--79.4094cM, likely due to endogamy. But my mother shares a full 160.0293--more than double that Jonathan shares with my uncle! Lesser differences can be seen between other relatives and my mother and uncle.

How about on my father's side? Fewer known relatives have tested, but here's what I have:

Name

Actual Relationship

Shared cM w/dad

Shared cM w/uncle

ISOGG average

Comments

George

1C1R

501.8542

433.2418

425.00

Ina(George's 1/2 1C)

Half 1C1R

326.0912

327.1629

212.50

Beth

2C

333.3482

320.6654

212.50

2 AJ grandparents

Dave(Beth's nephew)

2C1R

372.6452

241.0107

106.25

1 AJ grandparent

Table showing shared DNA between my father and paternal uncle and other known relatives

Cousins George and Ina and my paternal grandfather all shared the same grandfather; my grandfather and George also shared the same grandmother. While Ina shares almost exactly the same amount of DNA with both my father and uncle, there's a much larger difference in shared DNA when looking at George--my father shares nearly 70cM more DNA with George than does my uncle.

The other two people in the table are related to one another; Beth is second cousin to both my father and uncle, and Dave is her nephew. Even though Beth has only 2 Ashkenazic Jewish grandparents and Dave has one, the amounts of DNA they share with both my father and uncle are well above the ISOGG average. And while the amounts Beth shares with my father and uncle are pretty close, Dave shares quite a bit more with my father than he does with my uncle--over 130cM more! And even though Dave is a generation further removed from our shared ancestors than is Beth, my father actually shares more DNA with Dave than he does with Beth.

So while others' shared length of DNA segments can be a guide towards your relationship with someone with similar amounts of shared DNA, remember that it's just that--a guide. The shared amount can even vary between siblings. I wanted to demonstrate that my numbers
shouldn't be taken as absolutes for how much DNA would be shared for a
particular relationship in an endogamous situation.

4 comments:

Your data fits with my observations that close relationships (out to 1st cousins) are not that different in endogamous populations but from 2C1R and 3rds on out one starts to share significantly more than the expected amount of DNA. Have you seen the Paul, Tannenbaum et al paper on this done with family tree DNA data? http://www.surnamedna.com/?articles=differences-in-autosomal-dna-characteristics-between-jewish-and-non-jewish-populations